1. Field of the Invention
The present invention relates to an exposure device, and more particularly, to an exposure device in which a light beam emitted from a light source is main-scanned, and the light beam and a photosensitive material to be exposed are moved relative to one another such that sub-scanning is effected, and the photosensitive material is exposed.
2. Description of the Related Art
In an exposure device which scans/exposes a photosensitive material by using a deflector such as a polygon mirror or the like, the light emitting timing and the intensity of the emitted light of a semiconductor laser are controlled so as to correspond to the information or image to be recorded. While the light beam which is emitted from the semiconductor laser is deflected in the main scanning direction by the polygon mirror, the light beam exposes the photosensitive material so that information is recorded or an image is formed.
In such an exposure device, a photosensitive material is exposed from a start-of-main-scan point on the photosensitive material, i.e., from the point of the first exposure on the photosensitive material (the start of writing point), by using an SOS (start of scan) mirror and an SOS sensor. Namely, a light beam corresponding to a point outside of the image region on the photosensitive material is reflected at the SOS mirror and is incident on the SOS sensor. At this time, the SOS sensor outputs a predetermined signal to a microcomputer. The microcomputer to which the signal is input controls the light emitting timing of the semiconductor laser on the basis of the inputted signal, and the photosensitive material is exposed from the start of writing point in the main scanning direction on the photosensitive material.
However, because the polygon mirror in the above-described exposure device rotates at high speed, high frequency vibration is generated. A beat is generated due to the high frequency vibration of the polygon mirror and due to the natural vibration of the SOS mirror. The timing of the light beam reflected from the SOS mirror and incident on the SOS sensor deviates, and fluctuations (jitters) occur in the output timing of the signal outputted by the SOS sensor when the light beam is incident thereon. As a result, the start-of-main-scan point on the photosensitive material (the point of first exposure onto the photosensitive material) deviates. When the first exposure point deviates, irregularities in density in the subscanning direction are generated in the image recorded on the photosensitive material due to the multiple exposure effect of the photosensitive material. A drawback arises in that an image of appropriate image quality cannot be formed.
Further, due to the beat caused by the high frequency vibration of the polygon mirror and the natural vibration of the SOS mirror, the above-mentioned jitter fluctuates periodically. Accordingly, due to the periodic fluctuations of the jitter, the position at which writing of information onto the photosensitive material starts deviates periodically. When the start of writing position deviates periodically, for example, in a case in which the amount of jitter is large, the pixel positions, become disordered, and in a case in which the amount of jitter is small, the pixel positions are aligned. The disarray of the pixel positions and alignment of the pixel positions also occurs periodically. When this occurs, due to the multiple exposure effect of the photosensitive material, the image density when the pixel positions are aligned and the image density when the pixel positions are disordered are respectively different. As a result, a drawback arises in that irregularities in image density in the subscanning direction are generated periodically in the image recorded on the photosensitive material, and an image of the appropriate image quality cannot be formed.